The present invention relates generally to semiconductor microstructures, such as Micro-Electro-Mechanical Systems (MEMS) devices, along with fabrication and operational methods, and more particularly, to a high resolution in-plane fork gyroscope, and methods of manufacture and operation, which is preferably formed using a silicon-on-insulator (SOI) substrate.
Vibratory micromachined gyroscopes rely on Coriolis-induced transfer of energy between two vibration modes to sense rotation. Micromachined gyroscopes are increasingly employed in consumer and automotive applications, primarily due to their small size and low power requirements. However, they are yet to achieve performance levels comparable to their optical and macro-mechanical counterparts in high-precision applications such as space and tactical/inertial navigation systems.
Conventional MEMS vibratory gyroscopes have yet to achieve inertial grade performance. The requirements for inertial grade devices are rate resolutions and bias stabilities better than 0.1°/h. To achieve this, a vibratory gyroscope must attain very high quality factors (>30,000), large sense capacitances (>1 pF), large mass (>100 μg), and large drive amplitude (>5 μm).
The Brownian motion of the structure represents the fundamental noise-limiting component of a vibratory gyroscope. This is generally discussed, for example, by Ayazi, F., in “A High Aspect-Ratio High-Performance Polysilicon Vibrating Ring Gyroscope,” Ph.D. Dissertation, University of Michigan, Ann Arbor (2001), and Ayazi, F. and Najafi, K., in “A HARPSS Polysilicon Vibrating Ring Gyroscope” IEEE/ASME JMEMS, June 2001, pp. 169–179. By equating Brownian motion to the displacement caused by the Coriolis force, one can derive the mechanical noise equivalent rotation (MNEΩ) of the microgyroscope. This is expressed as
                              MNE          ⁢                                          ⁢          Ω                =                                            1                              2                ⁢                                  q                  Drive                                                      ·                                                            4                  ⁢                                      k                    B                                    ⁢                  T                                                                      ω                    0                                    ⁢                  M                                                              ⁢                      BW                                              (        1        )            
Equation 1 indicates that the mechanical noise floor varies inversely with the drive amplitude (qDrive), the square root of the resonant drive frequency (ω0), and square root of the effective mass in the sense direction (M). Matching the resonant frequencies of the sense and the drive mode improves this resolution by a factor of √{square root over (QSense)}.
This calls for innovative designs and advances in fabrication technology. It would be desirable to have an in-plane, solid-mass silicon tuning fork device that incorporates very high Q, a large mass per unit area, and in-plane matched-mode operation within a single framework—unlike previously reported tuning fork gyroscopes. Such conventional gyroscopes are discussed by Bernstein, J., et al., in “A Micromachined Comb-Drive Tuning Fork rate gyroscope,” Proceedings MEMS 1993, pp. 143–148, and Schwarzelbach, O., et al., in “New Approach for Resonant Frequency Matching of Tuning Fork Gyroscopes by Using a Non-Linear Drive Concept,” Proceedings Transducers 2001, pp. 464–467.
A number of US patents have been issued that generally relate to the present invention. These include U.S. Pat. No. 5,349,855, issued to Bernstein, et al. entitled “Comb drive micromechanical tuning fork gyro”, U.S. Pat. No. 5,488,863, issued to Mochida, et al. entitled “Angular velocity sensor making use of tuning fork vibration”, U.S. Pat. No. 5,505,084, issued to Greiff, et al. entitled “Micromechanical tuning fork angular rate sensor”, U.S. Pat. No. 5,728,936, issued to Lutz entitled “Rotary speed sensor”, U.S. Pat. No. 5,780,740, issued to Lee, et al. entitled “Vibratory structure, method for controlling natural frequency thereof, and actuator, sensor, accelerator, gyroscope, and gyroscope natural frequency controlling method using vibratory structure”, U.S. Pat. No. 5,780,739, issued to Kang, et al. entitled “Tuning fork type gyroscope”, U.S. Pat. No. 5,911,156, issued to Ward, et al. entitled “Split electrode to minimize transients, motor amplitude mismatch errors, and sensitivity to vertical translation in tuning fork gyros and other devices”, U.S. Pat. No. 5,920,012, issued to Pinson entitled “Micromechanical inertial sensor”, U.S. Pat. No. 5,945,599, issued to Fujiyoshi, et al. entitled “Resonance type angular velocity sensor”, U.S. Pat. No. 5,992,233, issued to Clark entitled “Micromachined Z-axis vibratory rate gyroscope”, U.S. Pat. No. 6,230,563, issued to Clark, et al. entitled “Dual-mass vibratory rate gyroscope with suppressed translational acceleration response and quadrature-error correction capability”, and U.S. Pat. No. 6,257,059, issued to Weinberg, et al. entitled “Micro-fabricated tuning fork gyroscope and associated three-axis inertial measurement system to sense out-of-plane rotation”.